docs: def export(self, nidm_version, export_dir):
          """
          Create prov entities and activities.
          """
          atts = (
              (PROV['type'], NIDM_ERROR_MODEL),
              (NIDM_HAS_ERROR_DISTRIBUTION, self.error_distribution),
              (NIDM_ERROR_VARIANCE_HOMOGENEOUS, self.variance_homo),
              (NIDM_VARIANCE_SPATIAL_MODEL, self.variance_spatial),
              (NIDM_HAS_ERROR_DEPENDENCE, self.dependance))
  
          # If the error covariance is independent then there is no associated
          # spatial model
          if self.dependance_spatial is not None:
              atts = atts + (
                  ((NIDM_DEPENDENCE_SPATIAL_MODEL, self.dependance_spatial),))
  
          # Create "Error Model" entity
          self.add_attributes(atts)